1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method of controlling transmission power for link-adaptive data transmission in a wireless communication system and user devices for the method.
2. Discussion of the Related Art
Recently, Bluetooth and wireless personal area network (WPAN) technologies have been developed, which form a wireless network between a relatively small number of digital devices in limited places such as homes or small companies to allow audio or video data to be exchanged between the devices. The WPAN can be used for information exchange between a relatively small number of digital devices in a relatively close distance, and enables low power and low-cost communication between the digital devices. IEEE 802.15.3 (Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for High Rate Wireless Personal Area Networks (WPANs)) approved on Jun. 12, 2003 defines specification of a MAC layer and a physical (PHY) layer of high rate WPAN.
FIG. 1 is a brief diagram illustrating an example of a wireless private access network (WPAN).
As illustrated in FIG. 1, the WVAN is a network configured between personal devices within a limited space such as home, and allows information to be exchanged between applications without seamlessness by configuring a network through direct communication between devices. Referring to FIG. 1, the WPAN includes two or more user devices 11 to 15, one of which acts as a coordinator 11. The coordinator 11 provides basic timing of the WPAN and serves to control quality of service (QoS) requirements. Examples of the user devices include computers, PDAs, notebook computers, digital TVs, camcorders, digital cameras, printers, mikes, speakers, headsets, bar-code readers, displays, and cellular phones. All digital devices can be used as the user devices.
The WPAN is not predesigned but is an ad hoc network (hereinafter, referred to as ‘piconet’) formed if necessary without assistance of a central infrastructure. A procedure of forming one piconet will be described in detail. The piconet starts as a random device that can be operated as a coordinator performs the function of the coordinator. All devices perform scanning before associating with the existing piconet or starting a new piconet. Scanning means that a device collects and stores information of channels and searches whether the existing piconet exists. A device that has been commanded from an upper layer to start a piconet forms a new piconet without associating with a piconet previously formed on a random channel. The device starts a piconet by selecting a channel having little interference based on data acquired during scanning and broadcasting a beacon through the selected channel. In this case, the beacon means timing allocation information, information of other devices within a piconet, and control information broadcasted by the coordinator to control and manage the piconet.
FIG. 2 is a diagram illustrating an example of a superframe used in a piconet. Timing control in the piconet is basically performed based on superframes. Referring to FIG. 2, each superframe starts by means of the beacon transmitted from the coordinator. A contention access period (CAP) is used to allow devices to transmit commands or asynchronous data based on contention. A channel time allocation period includes a management channel time block (MCTB) and a channel time block (CTB). The MCTB is a period where control information can be transmitted between a coordinator and a device or between devices. The CTB is a period where asynchronous data or isochronous data can be transmitted between a device and a coordinator or between other devices. For each superframe, the number, length and location of CAPs, MCTBs, and CTBs are determined by the coordinator and transmitted to other devices within the piconet through the beacon.
When a random device within the piconet needs to transmit data to the coordinator or other device, the device requests the coordinator to allocate channel resources for data transmission, and the coordinator allocates the channel resources to the device within the range of available channel resources. If the CAP exists within the superframe and the coordinator accepts data transmission in the CAP, the device can transmit data of small capacity through the CAP without being allocated with channel time from the coordinator.
If the number of devices within the piconet is small, since channel resources for data transmission from each device are sufficient, no problem occurs in allocation of channel resources. However, if channel resources are insufficient due to a large number of devices, or if data of large capacity such as moving pictures are transmitted, a problem may occur in that channel resources are not allocated to the other devices even though the other devices have data to be transmitted, whereby communication cannot be performed.
Also, during data communication between two or more devices that belong to the WVAN, communication quality may be deteriorated depending on the statuses of the devices, such as an obstacle occurring between the devices and interfering with communication, change of the distance or location between the devices, and interference caused by another neighboring device.
In this respect, various methods for efficiently performing data communication between devices constituting WVAN without any problem are being studied.